Nails have been decorated for a very long time. The staining of nails with henna has been traced back to the Ancient Egyptians.
Nowadays, the decoration and colouration of fingernails, toenails and artificial nail extensions rely, almost exclusively, on a number of well-established techniques: (1) applying a polymer based, pigmented nail lacquer in a solvent or solvents by a coating process and then subsequently evaporating the solvent or solvents; (2) mixing, applying and thermally curing two part (liquid and powder) acrylic components; and (3) applying by a coating process ethylenically unsaturated, UV radiation curable gels or top coats and subsequently curing the coating under a low intensity ultraviolet light source.
Most nail coatings are based on formulations that comprise a solvent and a film former (mentioned as (1) above). Although there has been some variation with the choice of solvent in the nail coating composition, the general components of modern nail coatings are very similar to those invented 80 years ago. There are only a small number of nail coatings manufacturers throughout the World, which might explain the stagnation in this particular field. Thus, modern nail coatings are essentially solvent borne films containing nitrocellulose, pigments, dyes and other optimizing substances.
The solvents most favoured in conventional nail coating compositions are polar organic ester and alcohol solvents for example butyl acetate, ethyl acetate and isopropanol. Toluene is still used in some countries, but because of its carcinogenicity, toluene is being phased out. These particular solvents have been chosen largely because of their ability to evaporate quickly and therefore allow the nail coating composition to dry quickly. Of course, nowadays there is a drive to improve the environmental-friendliness of products. A significant proportion of the content of conventional nail coating composition is solvent, which is evaporated when the nail coating composition is applied. This portion of the nail coating composition is therefore wasted. Additionally, the presence of the solvent in the nail coating composition leads to unnecessary weight in the transportation of the product to its point of sale.
In light of the introduction of regulations to reduce the use of volatile solvents in products, there have been attempts to reduce the usage of volatile solvents in nail coating compositions. For example, there have been attempts at using a water-based acrylic polymer emulsion. However, water-based compositions dry too slowly to be commercially viable.
UK Patent Application No. GB 2 452 566 is an earlier filed patent application of the same applicant. This document discloses a nail coating composition comprising a radiation curable nail coating substance and at least one dye and/or pigment dissolved and/or dispersed in the composition to impart a colour to the coating. The composition disclosed in this document does not use a volatile solvent in the composition, but instead uses an oil to disperse the pigment. The single example provided in this document is a radiation curable substance that comprises a thiol compound in an amount of about 2% w/w of the composition.
A study on these compositions suggested that the coating that resulted from the application and subsequent curing of this composition to the nail for 2 to 5 minutes was indistinguishable in appearance from that of a nail coated with a conventional solvent-based lacquer (when dried for a longer period of time).
Although the composition disclosed in the example of GB 2 452 566 exhibits an excellent thermal stability, resistance to premature cross-linking (i.e. cross-linking when in the bottle) and polymerisation and excellent shelf life properties, this product is rendered not commercially viable on account of its poor durability. Unfortunately, this document does not address important issue that the nail composition does not have an adequate level of chip resistance. Subsequent tests demonstrated that nail coating is prone to chipping after only two days. Thus, the adhesion of the coating produced by the composition of GB 2 452 566 is unsatisfactory in that the coating may become detached from the nail too quickly.
Thus, another desirable property of a nail coating composition is a high chip resistance once cured.
An article by Hoyle et al, Journal of Polymer Science; Part A; Polymer Chemistry; Vol. 42, 5301-5338 (2004) discloses that the photo polymerization of mixtures of multifunctional thiols and alkenes is an efficient method for the rapid production of films and thermoset plastics. It is disclosed that one of the major obstacles in traditional free radical photo polymerization is essentially eliminated in thiol-ene polymerizations because the polymerization occurs in air almost as rapidly as in an inert atmosphere.
This review article explains that general, thiol-ene systems polymerize by a free-radical chain mechanism involving two steps: an initial addition of the thiyl radical to the carbon of an ene functionality and a subsequent hydrogen abstraction of a thiol group by a carbon-centred radical to give a thiyl radical. Termination occurs by radical-radical coupling. The two-step process in results in the addition of a thiol group across an ene double bond:

It is also disclosed in this document that, although almost any type of alkene can participate in the reaction, methacrylate compounds are the slowest to react with the thiol component because of their relatively poor reactivity.
In order to improve chip resistance, conventional UV curable nail coatings utilise a multi-step/multi-product system. Thus, many nail coating systems require the application of several products in sequence to achieve the desired chip resistance. A UV curable undercoat treatment may first be applied to the nail to achieve a high degree of adhesion to the nail itself. Then a UV curable colour coating is applied in one or two layers to decorate the nail in a particular colour. Finally, a UV curable top coat of extremely glossy finish may be applied. Of course, the various products required for this system make the whole process a costly and laborious exercise. Further, mild abrasion may be required to remove the various UV cured nail coatings.
Alternatively, to achieve merely a transparent lightly coloured or glitter coating effect, UV curable coatings are often applied alone and then cured as in (3) above.
A problem with the use of UV curable compositions (mentioned as (3) above) is that UV curable compositions are inhibited from curing by air at the top surface of the coating. This means that to achieve a very high gloss and non tacky surface finish an additional subsequent process involving wiping away traces of incompletely cured material with a solvent soaked cloth or pad is also required.
US2003/0073753 discloses a composition for actinic radiation curable nail coatings and artificial nail tips comprising a BISGMA based urethane resin, an additional polymer, a photoinitiator and having a viscosity of greater than approximately 80,000 cps. There is also disclosed a single composition having 11.11% w/w of a specific tri-functional thiol in the composition. It is disclosed that the composition including the tri-functional thiol is “more tack-free” and that the coating is less brittle than when the composition does not include the thiol. However, the composition of this document does not demonstrate the desired properties that the coating is a dry-to-touch coating, i.e. a coating that, when applied to the nail and exposed to UV radiation, is entirely tack-free and does not require any wiping to remove the tacky outer layer that has not cured. Indeed subsequent testing by the applicant confirms that compositions having this level of thiol do not demonstrate the desired properties of an entirely tack-free, dry to touch composition, see Example 15 which includes a composition having 11.0% of a trifunctional thiol. The compositions of the present invention demonstrate the desirable properties of being entirely tack-free and dry-to-touch without the need to wipe off the tacky outer layer that has not cured and therefore represent a novel and inventive improvement over these prior art compositions.
US2007/0010617 discloses an aqueous dispersion for nail enamel. The aqueous dispersion includes a copolymer which is manufactured using an acrylate monomer and another radical-polymerisation unsaturated monomer in the presence of a thiol component. The actual nail enamel does not itself include a thiol component; the thiol component is utilised in the manufacture of the copolymer, and the copolymer is part of the aqueous dispersion for nail enamel. The copolymer that is produced is in the form of an aqueous dispersion which is then mixed with pigments and monomers to provide an aqueous nail enamel composition. Furthermore, although it is mentioned that the copolymer that is used in the aqueous dispersion for nail enamel can be manufactured using a thiol component having a formula of (HS—CH2—CH2—COO)n—R, wherein n can be 1 to 4, the examples only recite the manufacture of the copolymer using mono-functional thiol components; bis-, tris- and tetra-functional thiol components are not recited in the examples of US2007/0010617. In fact, the purpose of the thiol component is to chain-terminate the polymer with the thiol group and thus to control the emulsion polymerisation of the methacrylated monomers. The single thiol group will attach itself to the end of the chain by donating a hydrogen atom from the SH group and stop chain propagation at that end. Accordingly, the thiol component that is utilised in the manufacture of the copolymer can only be a mono-functional thiol as higher functional thiol components would be too reactive with the other acrylate monomer and radical-polymerisation unsaturated monomer components. Furthermore, there are always 10 parts or less thiol in the compositions of US2007/0010617, which means that the thiol component will always completely react with the methacrylate and unsaturated monomer components in the composition. Accordingly, the copolymer manufactured will never include a free —SH group for further reaction in the aqueous dispersion of the nail enamel.
U.S. Pat. No. 5,922,334 discloses a nail composition having a thiol component as a chain transfer agent. The chain transfer agent is used at a level from about 0.01 to 5 weight percent based on the total weight of the monomer. Examples of chain transfer agents include 3-mercaptopropionic acid and t-dodecyl mercaptan, i.e. thiol components having a single —SH group. There is no disclosure of thiol components having two or more —SH groups.
U.S. Pat. No. 3,896,014 discloses the use of an allyl oligomer in conjunction with a polythiol in a UV curable nail coating. However, the compositions in this document are based solely on polyenes, thiols and photoinitiators (i.e. the compositions do not include a monomer component). This means that the cured coatings will cross link very strongly and will be very difficult, if not impossible, to remove by a solvent soak off process if applied directly to the nail. The authors of U.S. Pat. No. 3,896,014 recommend the use of a tie coat layer with water swellable properties so that the complete nail coating can be removed in warm water. Thus, in order to obtain removal in warm water it is essential to apply the water soluble base coat. It is mentioned in U.S. Pat. No. 3,896,014 that the coating can be applied with or without a tie coat layer. However, there are problems when a tie coat is not used. If a tie coat layer is not used, the adhesion to the nail is poor (it will only last 12-24 hours before chipping occurs) and the only described method of removing the nail coating is to remove it by chipping (soaking off the nail coating is not an option). Accordingly, the only practical application of the UV curable nail coatings of U.S. Pat. No. 3,896,014 is to utilise a tie coat, which is time consuming and more costly.
In summary, therefore, the desired properties from a nail coating are:                good adhesion to the nail;        good impact resistance so that it does not chip off easily;        short drying time with a dry to touch surface finish;        high gloss shiny surface;        good compatibility with a vast array of conventional colorants; and        readily removable by the use of traditional nail polish removers.        
Objects of the Invention:
It is an aim of the present invention to provide a nail coating composition having good adhesion to the nail itself. It is therefore an aim of the present invention to provide a nail coating composition having a high chip resistance. It is therefore an aim of the present invention to provide a nail coating composition having a chip resistance of at least two days, preferably at least three days, preferably at least five days and more preferably at least ten days.
It is also an aim of the present invention to provide a nail coating composition having a short dry-to-touch time. It is therefore an aim of the present invention to provide a nail coating composition having a dry-to-touch time of 5 minutes or less, preferably 3 minutes or less and more preferably 2 minutes or less, e.g. no longer than 1 minute.
It is also an aim of the present invention to provide a nail coating composition having a high gloss finish.
It is also an aim of the present invention to provide a nail coating composition which may be applied to a nail in a single step (i.e. without the need for an undercoat or a top coat).
It is also an aim of the present invention to provide a nail coating composition which is compatible with most conventional colorants.
It is also an aim of the present invention to provide a nail coating composition which can be removed from the nail using conventional nail polish removers (i.e. without the need for abrasion).
It is also an aim of the present invention to provide a nail coating composition which avoids the use of volatile solvents.
The present invention achieves one or more, e.g. all, of the above listed aims.